Attention-deficit hyperactivity disorder is the most common developmental disorder in children, affecting anywhere between 3-5% of the world's school-going population. As the name suggests, kids with ADHD are hyperactive and easily distracted; they are also forgetful and find it difficult to control their own impulses.
While some evidence has suggested that ADHD brains develop in fundamentally different ways to typical ones, other results have argued that they are just the result of a delay in the normal timetable for development.
Now, Philip Shaw, Judith Rapaport and others from the National Institute of Mental Health have found new evidence to support the second theory. When some parts of the brain stick to their normal timetable for development, while others lag behind, ADHD is the result.
The idea isn't new; earlier studies have found that children with ADHD have similar brain activity to slightly younger children without the condition. Rapaport's own group had previously found that the brain's four lobes developed in very much the same way, regardless of whether children had ADHD or not.
But looking at the size of entire lobes is a blunt measure that, at best, provides a rough overview. To get an sharper picture, they used magnetic resonance imaging to measure the brains of 447 children of different ages, often at more than one point in time.
At over 40,000 parts of the brain, they noted the thickness of the child's cerebral cortex, the brain's outer layer, where its most complex functions like memory, language and consciousness are thought to lie. Half of the children had ADHD and using these measurements, Shaw could work out how their cortex differed from typical children as they grew up.
A child grows, their experiences manifest as connections between nerve cells and their cortex thickens. But during adolescence, the developing brain values efficiency over expansion and the cortex starts to thin, as unused connections are mercilessly trimmed. The growth of a child's brain into a teenager's is like the pouring of a block of clay that can then be sculpted away into the refined adult version.
In both groups of children, parts of the cortex peaked in terms of thickness in the same order, with waves of maturity spreading from the edges to the centre. The pattern was the same, but the timing wasn't.
Reference:Shaw, P., Eckstrand, K., Sharp, W., Blumenthal, J., Lerch, J., Greenstein, D., Clasen, L., Evans, A., Giedd, J., & Rapoport, J. (2007). From the Cover: Attention-deficit/hyperactivity disorder is characterized by a delay in cortical maturation Proceedings of the National Academy of Sciences, 104 (49), 19649-19654 DOI: 10.1073/pnas.0707741104